How browsers schedule and execute scripts can impact the performance of web pages. While techniques like <script defer>, <link rel=preload> (and others) influence script loading, knowing how browsers interpret them can also be helpful. Thanks to Kouhei Ueno, we now have an up to date summary of script scheduling in Chrome.

A Google Docs and HTML version of this table is also available.

Note: Loading priorities are not guaranteed to be consistent cross-browser so use this knowledge wisely and measure when unsure. Ideally, aim to delivery a great experience to the widest number of users possible.

If you’re a web developer wondering where you can see the “Loading Priority”, Chrome DevTools has…

Performance is a constant process, not a one-time checklist. It requires continuous monitoring and work. A useful workflow when investing in performance is Measure, Optimize & Monitor.

A few tips:

  • Link performance to your business goals. Help stakeholders measure how performance impacts the core business metrics they care about. (e.g conversions, bounce rates, brand perception)
  • Real-world performance is diverse. Measure performance on mobile devices & network connections common to your actual users. Understand your bottlenecks and their impact on user-centric performance metrics.

Improving Time-To-Interactive for on Desktop

tl;dr: There are no silver bullets to web performance. Simple static pages benefit from being server-rendered with minimal JavaScript. Libraries can provide great value for complex pages when used with care.

Netflix is one of the most popular video streaming services. Since launching globally in 2016, the company has found that many new users are not only signing up on mobile devices but are also using less-than-ideal connections to do so.

By refining the JavaScript used for’s sign-up process and using prefetching techniques, the developer team was able to provide a better user experience for both mobile and desktop…

If you’re building a web experience and want to stay fast, a performance budget can be critical. For success, embrace performance budgets and learn to live within them. Network & CPU limits on mobile can require asking hard questions like, “what is really important to my users?”

When we talk to Fortune 500 companies that put in the work to improve performance, perf metrics will often rapidly regress once they return to feature development. Performance budgets help teams prioritize features, optimizations and facilitate discussions on what is really important to users.

“Having a pre-defined ‘budget’ is a clear, tangible way…

Update: The Cost Of JavaScript In 2019 is now available to read.

Building interactive sites can involve sending JavaScript to your users. Often, too much of it. Have you been on a mobile page that looked like it had loaded only to tap on a link or tried to scroll and nothing happens?

Byte-for-byte, JavaScript is still the most expensive resource we send to mobile phones, because it can delay interactivity in large ways.

Tinder recently swiped right on the web. Their new responsive Progressive Web AppTinder Online — is available to 100% of users on desktop and mobile, employing techniques for JavaScript performance optimization, Service Workers for network resilience and Push Notifications for chat engagement. Today we’ll walk through some of their web perf learnings.

Pinterest’s new mobile web experience is a Progressive Web App. In this post we’ll cover some of their work to load fast on mobile hardware by keeping JavaScript bundles lean and adopting Service Workers for network resilience.

As we build sites more heavily reliant on JavaScript, we sometimes pay for what we send down in ways that we can’t always easily see. In this post, I’ll cover why a little discipline can help if you’d like your site to load & be interactive quickly on mobile devices.

tl;dr: less code = less parse/compile + less transfer + less to decompress


When most developers think about the cost of JavaScript, they think about it in terms of the download & execution cost. Sending more bytes of JavaScript over the wire takes longer the slower a user’s connection is.

At Google I/O this year, we introduced HNPWA (Hacker News Readers as PWAs) — a project aiming to demonstrate Progressive Web Apps can be implemented with popular JavaScript frameworks. Implementations follow a short specification but ultimately must demonstrate they can perform well on metrics like how soon a user can interact with the apps.

This is validated using Lighthouse on average mobile hardware (a Moto G4) using WebPageTest’s Faster 3G and Emerging Market 3G profiles.

We started off with apps written in Polymer, Vue, Preact, React, Svelte and Angular and thanks to the community have grown to 22 applications.


Authors: Treebo: Lakshya Ranganath, Chrome: Addy Osmani

Treebo is India’s top rated budget hotel chain, operating in a segment of the travel industry worth $20 billion. They recently shipped a new Progressive Web App as their default mobile experience, initially using React and eventually switching to Preact in production.

What they saw compared to their old mobile site was a 70%+ improvement in time to first paint , 31% improvement in time-to-interactive. and loaded in under 4 seconds over 3G for many typical visitors and on their target hardware. …

Addy Osmani

Eng. Manager at Google working on Chrome • Passionate about making the web fast.

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